The present invention relates to a semiconductor manufacturing apparatus wherein a wafer is aligned with a photomask or a reticle (hereinafter will be called simply "mask").
As one of the systems for aligning a mask and a wafer in a step-and-repeat type semiconductor circuit manufacturing apparatus, a TTL (through the lens) type die-by-die alignment system is known wherein the mask is aligned with each of the regions of the wafer through a projection lens for each of the shots. This system is advantageous in that the alignment operation is carried out for each of the shots so that a highly accurate alignment can be achieved.
Conventionally, in this type of system, two or more detecting means are located at different positions in order to detect the positional deviation between the mask and the wafer in X-direction, Y-direction and .theta.-direction (rotation). The mask and the wafer are placed into alignment with each other by the relative movement between them in response to the outputs of the detecting means.
In order to allow the detection of the positional relation between the mask and the wafer, they are provided with alignment marks. To save the space for those marks, such an alignment mark is often formed on a scribe line between chips or between zones, each of which is covered by a single shot (it is possible that one shot covers plural chips). This arrangement of the alignment marks involves a problem that, depending on the arrangement of the zones or shots, an automatic alignment operation cannot be effected on the basis of mark detections at plural different positions. Even if the alignment mark is located in the zone covered by a single shot, the same problem can occur. Therefore, there has been a problem that the arrangement which enables an automatic alignment results in the increase of the number of invalid zones, and therefore, the decrease of the number of the valid chips obtained from one wafer.
FIG. 1 shows an example of a zone or shot arrangement on a wafer. A zone P is the area covered by a single shot, that is, exposed by a single shot. In this example, four chips T are exposed by a single exposure shot. In the zone P, orthogonal scribe lines 1 and 1' are provided, and aligment marks are formed in the middle thereof.
It will be understood that the zone indicated by a referece numeral P' is void of a wafer alignment mark to be detected by a left-hand side view field of the detection optical system, so that the positional deviation can not be detected. Therefore, the automatic alignment operation can not be carried out between the mask and this zone P' of the wafer. As a result, this zone P' has to be an invalid zone.
After the automatic alignment between the mask and the wafer is accomplished, the wafer is exposed to a pattern of the mask so that the pattern is printed on the wafer, the wafer is then subjected to an etching and other operations for forming semiconductor circuits, during which the alignment marks can be damaged or partially failed. The partial failure or damage obstructs the production of proper alignment signals so that the zone having such an alignment mark can not be aligned. When this occurs, the apparatus gives up such a shot and advances to the next shot. This is disadvantageous in that the time required for attempting to detect the alignment mark has to be spent, more importantly, the chips in such a zone become rejects. In addition, there is a problem of pitch error caused by, for example, a reduction or an expansion of the wafer. This results in positional deviation between the wafer and the mask, which can not be obtained from the detection of only a part of the plural alignment marks (one of the alignment marks or one half of each of the alignment marks).
Furthermore, there is a problem of a particular positional deviation between the wafer and the mask (for example, rotational deviation), which can not be obtained by the detection of only a part of the plural alignment marks (one of the alignment marks or one half of each of the alignment marks).